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Due to the enormous progress in fabrication and characterization techniques, novel magnetic materials have been widely applied, for instance as magnetic sensors in cars (angle or position sensors). Magnetic sensors are made of thin layers with different magnetic properties. With the help of ion technology, scientists from Dresden were now able to shrink these multilayer systems down to one layer, retaining their magnetic properties. This discovery could make magnetic sensors even more powerful. The results have recently been published in the journal "Advanced Materials".

Progressive miniaturization is an important driving force for technological progress. Nowadays, magnetic multilayer systems for magnetic sensors are comprised of individual films, which are often only a few atomic layers thick. Scientists from the Leibniz Institute of Solid State and Materials Research (IFW) Dresden and from the Forschungszentrum Dresden-Rossendorf (FZD) picked up the well-known fact that it is not sufficient to reduce the thickness of the individual layers to miniaturize these systems. Instead of using multilayer systems a promising alternative is to combine the magnetic properties of the different layer materials within a single film. This goal has now been achieved by scientists from Dresden who produced an ultra-thin striped layer.

Traditional multilayer systems are made up of single layers consisting of hard magnetic and soft magnetic materials. Hard magnetic materials exhibit a stable magnetic configuration whereas the magnetization direction of soft magnetic materials can be easily controlled and thus reversed by applying a magnetic field. This effect is for instance used when magnetically stored data are read out by the read heads of hard disks. Read heads are in a way comparable to magnetic sensors like in cars or in other everyday applications, e. g. rotation controllers in hi-fi systems. Ultra-thin magnetic layer systems go back to the discovery of the giant magneto resistance effect (GMR) in ultra-thin magnetic films, for which Peter Grünberg and Albert Fert were awarded the Nobel prize last year.

In order to further miniaturize magnetic devices, intelligent combination of both hard magnetic and soft magnetic properties is essential. Researchers from FZD and IFW Dresden could now demonstrate for the first time that both material properties can be generated in a single film - in contrast to multilayer structures - by means of ion implantation on a micrometer scale. When observed from the top, the new structure shows a stripe pattern. The scientists found out that even in a single magnetic film the borders between both materials - also called domain walls - influence the magnetization reversal behavior. This discovery might enable more powerful magnetic sensors.

The new technology also opens up a route to imaging the domain walls by means of optical microscopy (Fig. 1, 2). In addition, the magnetization reversal behavior can be investigated as a whole (Fig. 3) and correlated to the magnetic domain configuration. In the near future, the scientists want to approach the nanometer regime in order to investigate the emerging physical effects at the largest level of miniaturization. Dr. Jürgen Fassbender, physicist at the FZD, explains: "We expect that at a certain feature size completely new effects arise."

Fig. 1

Figure by courtesy of WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim.False-color image of magnetization configuration of the stripe structure during the process of magnetization reversal. In principal, the magnetization can take on four different values, marked by corresponding arrows (non-irradiated area: red, blue, irradiated area: yellow, green).

Fig. 2

Left: Magnetic domain configuration during the magnetization reversal process of the 1 µm wide stripe array. Yellow arrows denote the direction and magnitude of the resulting total magnetization. Right: Sketch to visualize the magnetization configuration within the respective stripes. The low magnetization state (antiparallel magnetization configuration of adjacent stripes, upper image area) is separated from the high magnetization state (parallel configuration of adjacent stripes, lower image area) by so-called head-on domain walls (displayed in white). The antiparallel aligned magnetization directions in adjacent stripes are separated by 180° domain walls (displayed in red). These domain walls contain the energy which is the source of the magnetization reversal behavior.

Fig. 3

Magnetization reversal behavior of the 1 µm wide stripe array shown above with an applied field parallel (blue) and perpendicular (red) to the stripe direction. The magnetization reversal behavior along the stripe direction is characterized by a distinct two-step process. If only small magnetic fields are applied (± 0.5 kA/m) the resulting hysteresis curve is shifted with respect to the point of origin to negative field values. This is the characteristic feature of the lateral exchange spring effect.